1. Field of the Invention
The present invention relates to a process for electrolyzing an aqueous solution of an alkali metal chloride. More particularly, it relates to a process for producing an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride in a low cell voltage.
As a process for producing an alkali metal hydroxide by an electrolysis of an aqueous solution of an alkali metal chloride, it has been proposed to use an ion exchange membrane for producing an alkali metal hydroxide having high purity and high concentration instead of the process using an asbestos diaphragm.
On the other hand, it has been proposed to save energy in the world. From the viewpoint, it has been required to minimize a cell voltage in such technology.
It has been proposed to reduce a cell voltage by improvements in the materials, compositions and configurations of an anode and a cathode and compositions of an ion exchange membrane and a kind of ion exchange group.
In these processes, certain advantages can be considered. However, in most of these processes, the maximum concentration of the alkali metal hydroxide is not so high. In the case of higher concentration over the critical concentration, the cell voltage is seriously increased or the current efficiency is remarkably lowered. The maintenance and durability of the low cell voltage phenomenon have not been satisfactory for an industrial purpose.
It has been proposed to attain an electrolysis by a so called solid polymer electrolyte type electrolysis of an alkali metal chloride wherein a cation exchange membrane of a fluorinated polymer is bonded with gas-liquid permeable catalytic anode on one surface and a gas-liquid permeable catalytic cathode on the other surface of the membrane (U.S. Pat. No. 4,224,121).
This electrolytic method is remarkably advantageous as an electrolysis at a lower cell voltage because an electric resistance caused by an electrolyte and an electric resistance caused by bubbles of hydrogen gas and chlorine gas generated in the electrolysis, can be remarkably decreased which have been considered to be difficult to reduce in the conventional electrolysis.
In the process wherein the electrode is bonded to the cation exchange membrane, it is important how to smoothly and satisfactorily remove hydrogen gas and chlorine gas from the surfaces of the electrodes and cation exchange membrane by an electrolysis.
On the other hand, it has been proposed to decrease a cell voltage by using an oxygen-reduction (depolarized) cathode as the cathode and feeding an oxygen-containing gas such as air to react oxygen with water in the cathode so as to rapidly form hydroxyl ion. This cathode forms hydroxyl ion without generating hydrogen gas which causes higher electric resistance. Moreover, it has been proposed to produce an alkali metal hydroxide by bonding a liquid and gas permeable anode on one surface of the ion exchange membrane and using the oxygen-reduction cathode as a counter electrode. (U.S. Pat. No. 4,191,618).
In accordance with the process, the further decrease of a cell voltage is expected. It has been found that when the anode is directly brought into contact with the surface of the ion exchange membrane, the anode is directly brought into contact with hydroxyl ions reversely diffused from the cathode compartment, whereby high alkali resistance is required together with the chlorine resistance. Thus a special expensive substrate must be used for the anode. The life of the electrode is quite different from the life of the ion exchange membrane. When they are bonded, both of them are wasted in the life of one substrate. When an expensive noble metal type anode is used, this disadvantage reduces the advantage of the lower cell voltage.